Whats the miles on the 850? Evidence of history? And which engine ??

If its been looked after backed up by lots of receipts and you check it
carefully it sounds an ok buy. If its been abused, they can rapidly turn
into a big money pit!

The auto box is known not to be hugely long lived- about 150k is the max
(reason being no fluid changes are specified)

If its got aircon check it works- unless u are prepared to live without it-
evaporators very commonly leak and its a dash out job to replace.

Otherwise,

Sagging rear suspension / sloppy handling (worn springs / shocks)
Clonking from front (wishbones)
Tappetty engine (lack of oil changes and worn followers - $$$$)
Dirty coolant (headgaskets can fail- $$$$)
ABS light on (ATE computer is known to be weak- $$$$ again!)
Drivers door check strap breaking away from panel, also worn hinge pins
(cheap to fix)
Electric window switch pack bust (new ones are 70quid)

Tim..

Try Direct Line for the loan (currently 5.9%). You will find the 850 more
expensive to run than the 945 S, especially in the area of front tyres.
However if you wish to buy the car don't let me stop you as you will
probably get as much satisfaction from the 850 as you have done from your
945 S.

All the best, Peter.

700/900/90 Register Keeper,
Volvo Owners Club (UK).